Some communication systems transmit and receive data overt redundant communication links, in order to provide protection against equipment failures and adverse channel conditions. For example, Ericsson LM (Kista, Sweden) offers a microwave link product line called MINI-LINK, which supports such protected configurations.
U.S. Pat. No. 6,611,942, whose disclosure is incorporated herein by reference, describes a method of protecting the transmission of cells in a telecommunication system. On the transmitter side, two identical flows of cells are transmitted on two distinct physical links. Cells serving as markers, and thus delimiting blocks of cells or sets of blocks of cells, are inserted regularly into each of the flows at the transmitter. On the receiver side, the two flows of cells are received and the block or group of blocks from the flow having the fewer transmission errors is selected.
U.S. Pat. No. 5,631,896, whose disclosure is incorporated herein by reference, describes a path switching method without bit loss. The same digital line signals on a working path and on a protection path are continuously monitored independently for bit errors. If a bit error occurs in the working path and no bit error occurs in the protection path, a switching trigger is produced and a switching operation from the working path to the protection path is performed on a data block basis. Only correct data are transferred to downstream apparatuses. The method uses data blocks of one frame length with an indicator for bit error checking placed at the beginning or top of the block.
Other communication systems, commonly referred to as Multiple-Input Multiple-Output (MIMO) systems, transmit and receive simultaneously using multiple transmit and receive antennas. Some MIMO techniques are used for providing spatial multiplexing of signals. For example, U.S. Pat. No. 6,937,592, whose disclosure is incorporated herein by reference, describes a wireless communication system that adapts its mode of operation between spatial multiplexing and non-spatial multiplexing in response to transmission-specific variables. In an embodiment, a subscriber unit includes mode determination logic that is in communication with the subscriber unit and with a base transceiver station. The mode determination logic determines, in response to a received signal, if a subscriber data stream should be transmitted between the base transceiver station and the subscriber unit utilizing spatial multiplexing or non-spatial multiplexing.
U.S. Patent Application Publication 2007/0010209, whose disclosure is incorporated herein by reference, describes a control method used in a communication system that includes multiple transmission and reception antennas. When a reception side detects a state change between a normal state and an abnormal state in any reception section among a plurality of reception sections, the reception side calculates a number equal to or less than the number of normal reception sections, and transmits information of the calculated number to the transmission side. The transmission side matches a number of transmission sections that are to be operated accordingly.
Other communication systems transmit phase-shifted replicas of a transmitted signal from multiple antennas to produce a directional, high-gain transmission. These techniques are often referred to as Beam Forming (EF) techniques.